They're useless upgrade. Either you go to lvl 8 or keep lvl 6 longer. I upgraded them first before the wizard towers and regretted it. The dps increase wasn't apparent. They quickly canceled by hogs and air units. Even lvl 6 goblins can outrun them. While the price is far less than wizard tower of the same level, it requires the same amount of time.
I prefer air defenses and x-bows after lvl 8 walls, since more attacks come from air units.




